i965: Fix asynchronous mappings on !LLC platforms.
When using a read-only CPU mapping, we may encounter stale buffer contents. For example, the Piglit primitive-restart test offers the following scenario: 1. Read data via a CPU map. 2. Destroy that buffer. 3. Create a new buffer - obtaining the same one via the BO cache. 4. Call BufferSubData, which does a GTT map with MAP_WRITE | MAP_ASYNC. (We avoid set_domain for async mappings, so no flushing occurs.) 5. Read data via a CPU map. (Without explicit clflushing, this will contain data from step 1!) Otherwise, everything ought to work, keeping in mind that we never use CPU maps for writing - just read-only CPU maps. This restores the performance gains after Matt's revert in commit 71651b31. v2: Do the invalidate later, and even when asking for a brand new map. v3: Add more comments from Chris. Reviewed-by: Chris Wilson <firstname.lastname@example.org>
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